This invention relates to semiconductor processing and, more particulary, to a method of controlling the edge profile of chemically etched metal lines.
The natural tendency of a chemical etching step, when it is necessary to form fine line metal structures, is to produce a structure which, in cross section, is narrower at the top and wider at the bottom. This characteristic is due to the essentially isotropic nature of the chemical etching process. Such a process is illustrated in FIGS. 1a and 1b where a metal layer to be etched 14 is formed on substrates 10 and 12. These substrates may, for example, be a substrate 10 of undoped Gallium Arsenide (GaAs) and a layer 12 of doped GaAs. Metal layer 14 is covered by a photoresist mask 16 whereby the portion of layer 14 under mask 16 defines the gate member. Assuming perfect adhesion of the masking layer, it should be noted that if the etching process were to continue to the point where the upper surface of layer 12 is exposed, layer 14 would be undercut under mask layer 16. Further, it would be seen that the narrower part of the etched metal line is that portion of layer 14 in contact with layer 16 as shown in FIG. 1b.
In defining the gatelength in a Schottky barrier Field Effect Transistor, one would prefer to have the narrower part of the etched metal line in direct contact with the doped GaAs layer rather than at the top. The gatelength being defined as the width of the metal line in contact with the doped GaAs layer 12.